In a general wireless access system, only one carrier is generally considered even though uplink (UL) and downlink (DL) bandwidths are differently configured. For example, a wireless communication system in which the number of carriers constituting each of UL and DL is one and a UL bandwidth and a DL bandwidth are generally symmetrical may be provided based on a single carrier.
In the International Telecommunication Union (ITU), an IMT-Advanced candidate technique demands support of an extended bandwidth compared with a conventional wireless communication system. However, it is not easy to allocate frequency of a wide bandwidth except in some areas of the world. Accordingly, a carrier aggregation (also called bandwidth aggregation or spectrum aggregation) technique for producing an effect as if a broadband is logically used by physically aggregating a plurality of bands in the frequency domain has been developed as a technique for efficiently using a fragmented narrowband.
Carrier aggregation has been introduced to support increased throughput, prevent cost increase caused by introduction of a broadband RF element, and ensure compatibility with existing systems. Carrier aggregation refers to a technique for exchanging data between a User Equipment (UE) and a Base Station (BS) through plural aggregates of carriers in the unit of a bandwidth defined in an existing wireless access system (an LTE system in case of an LTE-A system, or an IEEE 802.16e system in case of an IEEE 802.16m system).
Here, a carrier of the unit of a bandwidth defined in an existing wireless communication system may be called a Component Carrier (CC). Carrier aggregation may include a technique supporting a system bandwidth of up to a maximum of 100 MHz by aggregating a maximum of 5 CCs even if one CC supports, for example, a bandwidth of 5 MHz, 10 MHz, or 20 MHz.
When using the carrier aggregation technique, data can be simultaneously transmitted and received through multiple UL/DL CCs. Hence, a UE is capable of monitoring and calculating all CCs.